Resonance contact scanning force microscope

ABSTRACT

The resonance contact scanning force microscope includes a reflective cantilever arm which is oscillated at a high harmonic of the resonance frequency of the cantilever arm, while the probe tip is maintained in substantially constant contact with the surface of the specimen. The motion of the free end of the cantilever arm is measured, to generate a deflection signal indicative of the amount of actual deflection of the probe tip. The method and apparatus permit high speed scans and real time imaging of the surface of a specimen with a substantial reduction in noise normally arising due to tip-surface interaction and acoustic noise.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to scanning force microscopes, and moreparticularly concerns a scanning atomic force microscope with anoscillating cantilever probe and a modulated resonance contact mode ofoperating the microscope for imaging surface contours of a specimen.

2. Description of Related Art

Scanning force microscopes, also known as atomic force microscopes, areuseful for imaging objects as small as atoms. Scanning force microscopyis closely related to scanning tunneling microscopy and the technique ofstylus profilometry. In a scanning force microscope, a laser beam istypically deflected by the free end of a reflective lever arm to whichthe probe is mounted, indicative of vertical movement of the probe as itfollows the contours of a specimen. The deflection of the laser beam istypically monitored by a photosensor in the optical path of thedeflected laser beam, and the sample is mounted on a stage moveable inminute distances in three dimensions. The sample can be raster scannedwhile the vertical positioning of the probe relative to the surface ofthe sample is maintained substantially constant by a feedback loop withthe photosensor controlling the vertical positioning of the sample.

The interactive forces between the probe and surface of the specimenchange at different distances. As the probe approaches the surface of anuncontaminated specimen, it is initially attracted to the surface byrelatively long range attractive forces, such as van der Waals forces.As the probe tip approaches further, repulsive forces from the electronorbitals of the atoms on the probe tip and the specimen surface becomemore significant. Under normal ambient conditions, the surface of aspecimen will also be covered by a thin contamination layer, typicallycomposed of water and other ambient contaminants, and contaminantsremaining from production of the specimen. The thickness of thecontamination layer can vary due to humidity and specific ambientconditions, but is generally between 25 and 500 Å. This contaminationlayer can also have an interactive effect on the probe tip. As the probetip approaches the contamination layer of a specimen, capillary surfaceforces can strongly attract the probe tip toward the surface of thespecimen. When the probe tip is being retracted from the surface of thespecimen, the capillary attraction forces can also strongly resistretraction of the probe tip from the surface of the specimen.

In conventional non-modulated modes of operating atomic forcemicroscopes, where the lever arm is not oscillated, output from thedetector monitoring the deflection of the reflective probe lever arm istypically used as feedback to adjust the position of the probe tip tomaintain the interactive forces and distance between the probe tip andspecimen surface substantially constant. In a conventionalnon-modulated, DC-contact mode of operation, the detected displacementof the probe is used in a feedback loop to adjust the position of theprobe so that the force between the probe and the specimen surfaceremains substantially constant. It has been observed that in anon-modulated contact scanning mode, high rates of scanning, i.e. atfour scan lines per second over a 50 micron range, can result in ahydroplaning effect, with the probe tip skimming over the surface of acontaminant layer, causing an unusual amount of noise to be present inthe output signal.

In modulated modes of operating a scanning force microscope, thereflective lever arm is typically mounted to a piezoelectric ceramicmaterial which can be driven by an alternating voltage to cause thelever arm and the probe tip to oscillate at a desired frequency. Inmodulated "non-contact" and "intermittent contact" scanning modes, asthe oscillating probe tip approaches the surface of the specimen, boththe amplitude and phase shift of the probe relative to the drivingoscillator are perturbed by the surface forces. Measurements aretypically made of the average cantilever amplitude or the shift in phaseof the cantilever relative to the driven oscillation, in order tomonitor the interaction of the tip with the attractive and repulsiveforces of the surface of the sample, generally due to a contaminantlayer on the surface of the sample, in ambient, open air conditions.Either the change in amplitude or the change in phase can typically beused in a positioning feedback loop.

In a conventional high amplitude resonance modulation mode, in which theprobe is oscillated at its resonant frequency, typically at 50-500 kHz,at a high amplitude of from 100 to 1,000 Å, the probe has intermittentcontact with the surface of the specimen, rapidly moving in and out ofthe contamination layer. In this mode, the topographical image is notsignificantly affected by the contamination layer, since the proberapidly penetrates this layer. Either the probe or the sample can bedamaged in this mode, which is more appropriate for imaging softspecimens. In a conventional low amplitude resonance mode, in which theprobe tip is also typically oscillated at it resonance frequency at from50-500 KHz at a low amplitude, the probe remains within thecontamination layer, in the attractive region. However, since thecontamination layer can change, due to warming of the specimen, changesin humidity or other ambient surface conditions, images made with inthis mode of operation can also change unpredictably.

Resonance modes of operation also present special problems, in thatchanges in amplitude and phase during oscillation of the lever arm dueto long and short range forces occurring between the tip and the surfaceof the sample are most greatly affected when the frequency is at or nearthe fundamental resonance frequency. At resonance, the oscillation isquickly damped when the probe tip is at or near the sample surface. Thequality factor, Q, of the oscillating lever arm at resonance furtherincreases the effect of the interacting surface forces on the amplitudeand phase shift. For a single optical lever arm made of silicon (100microns long, 15 microns wide, 6 microns thick), the resonance frequencyis about 300 Khz, and the Q factor is well over 100 in air. However,operation of a scanning force microscope with a lever arm having a highQ factor in "non-contact" mode at the resonance frequency can cause"ringing" problems, reducing frequency response. Consequently,conventional resonance modes of operation typically result in lowresolution imaging of the surface of a specimen.

It has been found that the quality of scanning force microscope imagesfor small scan ranges, i.e. less than about 1 micron, and with surfaceshaving small features, i.e. less than about 20 nm, is limited by noisefrom the dynamics of tip-surface interaction and acoustic noise.Acoustic noise combined with possible resonance feedback from a normallaboratory environment can also result in reduced image resolution,particularly when a probe is oscillated at lower frequencies, such asfrom 10-100 KHz. At high scan rates, such as five scan lines per secondover a 400 nm range, acoustic noise can be as much as 10 Å in comparisonto features of from 10-15 Å. Factors such as feedback control settings,the scan rate, and the frequency characteristics of the lever arm of theprobe can affect the amount of noise encountered, but tip-surfaceinteraction noise and acoustic noise typically can easily exceed designperformance of the microscope, causing streaks in the images ofspecimens which are much longer than the effective tip-contact radius.

It would be desirable to provide a way of overcoming problems of noiseand differences in interaction of the probe tip with a contaminationlayer to provide for high resolution imaging at high scan rates. Thepresent invention addresses these needs.

SUMMARY OF THE INVENTION

Briefly, and in general terms, the present invention provides for ascanning force microscope with an oscillating cantilever probe, and aresonance contact method for using the microscope which allows highspeed scanning of surface contours of a specimen for producing an imageof the specimen surface contours. The invention also provides forreduction of noise in specimen images due to tip-surface interaction andacoustic noise, which is useful for improved resolution from both highand low speed scans.

The invention accordingly provides for a resonance contact scanningforce microscope for examining surface contours of a specimen. Themicroscope has a body including a reflective cantilever arm having afirst end secured to the body. The cantilever arm is oscillated at adesired frequency which is preferably a very high resonance harmonic ofthe resonance frequency of the cantilever arm, and is preferably betweenabout 0.1 and 2 MHz. A probe is secured to the free end of thecantilever arm, and includes a probe tip adapted to maintainsubstantially constant contact with and follow the surface contours ofthe specimen with a substantially constant amount of force. Scanningmeans are provided for scanning the specimen relative to the probe tip.Means are provided for measuring the deflection of the free end of theoscillating cantilever arm, operating to generate a deflection signalindicative of the deflection of the probe at the free end of thecantilever arm. The means for measuring the deflection of the free endof the oscillating cantilever arm preferably comprises a light sourcemeans mounted to the body for producing a focused beam directed at anddeflected by the free end of the reflective cantilever arm, withphotosensor means mounted to the body for receiving the reflected beamdeflected by the cantilever arm. In one presently preferred embodiment,the light source is a laser light source.

The invention also provides a method for oscillating the probecantilever arm of the scanning force microscope in a resonance contactmode for examining surface contours of a specimen. The method generallyinvolves oscillating the cantilever arm at a desired frequency,preferably at a very high resonance harmonic of the resonance frequencyof the cantilever arm of the microscope, and preferably between about0.1 and 2 MHz. Deflection signal data indicative of the surface contoursof the specimen are then generated based upon the measured deflection ofthe free (probe) end of the cantilever arm. The signal data can then beused for creating an image of the surface contours of the specimen.

These and other aspects and advantages of the invention will becomeapparent from the following detailed description, and the accompanyingdrawings, which illustrate by way of example the features of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a resonance contact scanning forcemicroscope of the present invention;

FIG. 2 is an exploded diagrammatic perspective view illustrating thespatial relationships of major elements of the resonance contactscanning force microscope of the present invention;

FIG. 3 is an enlarged, exploded view showing the mounting of the opticallever arm assembly of the scanning force microscope of the invention;

FIG. 4 is an enlarged perspective view of the integral support member,cantilever arm, and probe tip of the optical lever arm assembly of thepresent invention;

FIG. 5 is an illustration of motion of the oscillating cantilever armwhile the probe is maintained in contact with the surface of a specimenin the method of the invention;

FIG. 6 is a schematic diagram of a feedback control circuit for theresonance contact scanning force microscope of the present invention;

FIG. 7 is a schematic diagram of an alternate feedback control circuitfor use with the resonance contact scanning force microscope of thepresent invention;

FIG. 8 is a diagrammatic side view of alternate embodiment of aresonance contact scanning force microscope of the invention placed on asubstrate for examination;

FIG. 9 is a cross-sectional elevational view of the resonance contactscanning force microscope of FIG. 8; and

FIG. 10 is an enlarged, exploded view showing the mounting of theoptical lever arm assembly of the scanning force microscope of FIG. 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In non-modulated contact scanning modes of operation used with scanningforce microscopes, high scan rates can produce noise in specimen imagesdue to a hydroplaning effect, acoustic noise, and tip-surfaceinteraction effects. In modulated contact scanning modes, at or near theresonance frequency of the probe lever arm, oscillation of the probelever arm is quickly damped when the probe tip is at or near the samplesurface. Operation of a scanning force microscope with a lever armhaving a high Q factor at the resonance frequency can also cause"ringing" problems, reducing frequency response.

The present invention provides for noise reduction, at high or low scanrates of a scanning force microscope, by oscillating the probecantilever arm of the microscope at a frequency which is a very highresonance harmonic of the resonant frequency of the cantilever arm whilethe probe tip is maintained in substantially constant contact with thesurface of the specimen to overcome these problems.

As is illustrated in the drawings, and with particular reference toFIGS. 1, 2 and 5, the invention is embodied in a resonance contactscanning force microscope 10 for examining the surface contours 11 of aspecimen 12, which is generally less than 1/2 inch in diameter and 0.1to 2 mm thick. The surface of the specimen is typically covered with acontamination layer 14 in normal ambient conditions, which represents anattractive region primarily due to surface attraction of the probe tipof the scanning force microscope.

Referring to FIGS. 1 and 2, in one presently preferred embodiment, theresonance contact scanning force microscope of the invention includes astationary body 16 having a removable lower base 18 secured to the bodyby screws or bolts, with a chamber 20 for receiving the specimen. Thebody is typically supported by a stationary support (not shown) alsosupporting a scanning means 22 for mounting of the specimen forexamination, raster scanning the specimen in X and Y directions relativeto the body, as shown by the arrows, that is, horizontally in twodimensions or degrees of freedom, and for moving the specimen in avertical or Z dimension or degree of freedom relative to the body, asshown by the arrow, and as will be further explained. The scanning meansis thus stationarily mounted with respect to the body of the microscope,and can also be secured to the body. The scanning means can for examplecomprise a piezo tube 24 with a support stage 26 for the specimen at thedistal end 28 of piezo tube, which is typically driven in the X, Y and Zdimensions by electrical drive voltage signals from a control unit, asis further discussed below.

The stage also may include a magnet 30, to allow a specimen to bemounted by adhesion onto a small magnetic steel plate which can thus bemagnetically secured on the top of the stage, allowing for theconvenient interchange of specimens to be examined by the instrument.

As is best illustrated in FIGS. 1-4, the microscope includes an opticallever arm assembly 32 having a first end 34 secured to the body, and afree end 36. The optical lever arm assembly is preferably secured to thebody of the microscope by a half washer member 38 of magnetic steel,which can be readily magnetically secured to one or more magnetizedportions 40 mounted to the microscope body, and currently preferablydisposed in a cantilever holder member 41. An integral cantileversupport member 42 is also currently preferably mounted to a centralportion 44 of the half washer member, extending to the open middleportion 46 of the half washer member. A reflective cantilever arm 48 iscarried by the integral cantilever support member. Referring to FIGS. 1and 6, an oscillation drive means 56 is also preferably provided, andcurrently preferably comprises a piezo-ceramic transducer 58, securedbetween a piezo holder member 59, which is mounted to the body or to thebase, and the cantilever holder member. The piezo-ceramic transducer ispreferably driven by oscillating voltage conducted by electrical lines60 connected to a voltage oscillator 62, for oscillating the cantileverarm at a desired frequency. Alternatively, the specimen can beoscillated relative to the cantilever arm, by control of the scanningmeans by the oscillator, as is shown in FIG. 7.

The oscillating frequency of the cantilever is preferably selected to bea very high harmonic of the fundamental resonance frequency of thecantilever arm in a range of from about 0.1 to about 2 MHz, and iscurrently preferably above about 0.4 MHz. In actual practice, the highharmonic frequency is selected by varying the oscillation frequencyuntil the oscillation frequency reaches a high harmonic and theamplitude of noise from the oscillating cantilever arm reaches aminimum. The minimum frequency should be selected to effectively preventintroduction of audio frequencies up to 20 KHz into the feedback loop,and the preferred minimum of about 100 KHz has been found to beeffective for this purpose. Also, if amplitude of oscillation of thecantilever arm is sufficiently small, the noise reduction effect willnot occur. Therefore, in order to maintain the amplitude of oscillationabove an effective minimum, the cantilever arm is typically driven tooscillate at an amplitude above about 10 Å, and generally in the broadrange of between about 10 Å and 300 Å.

The reflective cantilever arm 48 is currently preferably formed in theshape of a triangle from first and second arms 50a, 50b, and ispreferably secured at one end to the oscillator drive means 56 at thefree end 52 of the integral cantilever support member, and the arms ofthe cantilever arm are joined together at their free ends 54.

The reflective cantilever arm preferably is relatively soft, having arelatively low force modulus, generally in the range of about 0.05 to0.1 nano-Newtons/nanometer, to allow the cantilever arm to oscillate athigh frequencies, while maintaining the probe tip in contact with thesurface of the specimen. The arms of the cantilever arm are typicallyabout 18 microns wide, about 0.5 to 1.0 microns thick, typically about0.6 microns thick, and about 200 microns long, and are secured to theintegral cantilever support member about 120 microns apart. Thecantilever arm so constructed typically has a free air resonancefrequency of about 10 KHz. The silicon nitride material (available fromPark Scientific Instruments, Topometrix, and Digital Instruments) fromwhich the cantilever arm is made allows the cantilever arm to bow andflex as much as 30°, amplifying the deflection of the laser beam. Theintegral cantilever support member and the reflective cantilever arm areso small that they are most conveniently etched from silicon nitride,although other materials such as silicon which can be etched or lendthemselves to fine machining and which can provide a reflective surface,such as are well known to those skilled in the art of manufacturing ofintegrated circuit chips, may be suitable as well.

Attached to the free end 54 of the cantilever arm is a probe means 66including a distal needle-like probe tip 68 adapted to maintain contactwith and follow the surface contours of the specimen. The microscopepreferably also includes a processing unit with feedback control means70 for driving the piezo tube in the vertical or z dimension as theprobe tip traverses the contours of the specimen, to maintain asubstantially constant force of the probe tip against the surface of thespecimen even as the cantilever arm oscillates.

With reference to FIGS. 1, 2, 6 and 7, deflection measuring means 72 arealso mounted to the body of the microscope for measuring deflection ofthe probe at the free end of the cantilever arm as the probe follows thecontours of the surface of the specimen. In a preferred embodiment, thedeflection measuring means includes a laser light source means 74 suchas a laser diode with associated optics, mounted in the upper portion ofthe body for producing a focused laser beam 76 directed at and deflectedby the free end of the reflective cantilever arm. One preferred laserdiode is a 3 milliwatt laser diode which produces a beam in the 670 nmrange, and is commercially available. An opening 78 or transparentwindow is provided in the removable base to allow the laser beam to passthrough to the cantilever arm. Adjustment screws 80 may be provided foradjusting the alignment and aiming of the laser light source, mounted inthreaded access ports 81 provided in body. A reflective means such asthe planar mirror 82 is preferably mounted in the interior of the bodyat a distal end of an adjustment screw (not shown) through a threadedaccess port to reflect the deflected beam 88 to a photosensor 90 mountedto the body of the microscope for receiving the deflected laser beam.The photosensor preferably generates an electrical deflection signal 102in response to the deflected laser beam indicative of an amount ofdeflection of the laser beam by the cantilever arm.

As is shown in FIG. 2, the photosensor is preferably mounted to the bodyof the microscope to receive the deflected laser beam throughphotosensor port 92 in the body, and is typically formed as an array offour photodetectors 94, in which the top pair 96a is coupled to providea combined signal, and the bottom pair 96b is coupled to provide acombined signal, with the two combined signals being received bypreamplifiers 98a,b and differential amplifier 100 to provide a combineddeflection signal 102. The deflected laser beam is typically targeted ata central point between the top and bottom portions of the photosensor,and the combined deflection signal 102 is processed in the processingunit 70 for generating feedback to the z-piezo of the scanning means,and topographical position signal output data for imaging the surface ofthe specimen. Although the laser beam deflected by the rapidlyoscillating cantilever arm will also oscillate at the selected highharmonic resonance frequency, because the bandwidth of the photosensoris about 20 KHz, the photosensor will essentially measure a mediandeflected laser beam excursion, and any effect of attenuation of thedeflection signal due to the oscillation will be very small. Theprocessing unit provides feedback to maintain a substantially constantamount of force of the probe tip in constant contact with the surface ofthe specimen, as is illustrated in FIG. 5. Although the cantilever armoscillates rapidly, varying the amount of force of the probe tipactually applied at any given instant, due to the limited bandwidth ofthe photosensor, the force detected is also a median value, which iskept substantially constant. The processing unit is electricallyconnected to the scanning means 22 for raising and lowering the specimenwith respect to the probe tip for increasing or decreasing the force ofthe probe tip against the specimen surface to maintain the substantiallyconstant amount of force of the probe tip against the specimen surface.

With reference to FIGS. 8-10, illustrating an alternate free standingtype of scanning force microscope implementing the principles of theinvention, the free standing resonance contact scanning force microscopeof this embodiment has a stationary body 212 including a lower base 214.The base is secured to the body by screws or bolts, with adjustablemotor driven legs 216a,b,c for supporting the body of the microscope ona substrate 213 and moving the body of the microscope in a verticaldimension relative to a specimen 218 mounted with respect to thesubstrate. Each of the motor driven legs includes an optically encodedscrew drive motor 217 connected to a control unit, as is describedlater, which coordinates the operation of the legs in response toposition signals from the optically encoded screw drive motors, so thatthe motors operate in unison to raise and lower the microscopeuniformly. The specimen to be examined can in fact consist of a portionof the substrate, and can therefore in principle be of any size orweight, such as the wing of an aircraft, or a desk top, for example. Thebase of the microscope preferably includes a middle bore 220 forextension of the sensor head to the specimen to be examined.

A scanner assembly shell 228 is preferably mounted to the upper side ofthe base. The upper end of the scanner shell assembly provides a sitefor mounting a pivot 234 for the body of the generally cylindrical innersensor assembly, or kernel, which includes a laser light source andsensor head assembly 236.

The sensor head assembly 236 preferably includes an optical lever armassembly 240 secured to the body of the inner sensor assembly, andpreferably includes a half washer member 242 of magnetic steel,magnetically secured to a magnet 244 secured to the body of the innersensor assembly. An integral cantilever support member 245 is mounted toa central portion of the half washer member, extending to the openmiddle portion of the half washer member. A relatively soft, reflectivecantilever arm 246 formed in the shape of a triangle from first andsecond arms is secured at one end to the free end 247 of the integralcantilever support member and joined together at their free ends 250.Attached to the free end 250 of the cantilever arm is a probe means 260including a distal needle-like probe tip 262 adapted to contact andfollow the surface contours of the specimen.

A laser light source means 270 such as a laser diode with associatedoptics, is mounted in the upper portion of the body for producing afocused laser beam 272 directed at and deflected by the reflectivecantilever arm. An opening 273 is provided in the removable base toallow the laser beam to pass through to the cantilever arm. A reflectivemeans such as the planar mirror 280 is preferably mounted in theinterior of the body at a distal end of an adjustment screw through athreaded access port (not shown) to reflect the deflected beam 286 to aphotosensor 288 mounted to body for receiving the deflected laser beam.

As explained previously, with reference to FIGS. 2 and 6, thephotosensor preferably generates an electrical output signal in responseto the deflected laser beam indicative of the degree of deflection ofthe laser beam by the cantilever arm.

Referring to FIGS. 8 and 9, the mechanism for raster scanning the sensorhead preferably includes a pair of stacked piezo drivers 300 disposed inthe scanner shell assembly oriented horizontally at right angles to eachother for low resolution or large scale x and y raster scanningmovements ranging approximately from zero to 400 microns, andcorresponding opposing coil compression springs 302. The large motionhorizontal piezo drivers 300 and the opposing coil springs are mountedbetween the inner scanner assembly and the scanner shell in push rods296 having push rod chambers for containing and securing one end of thepiezo drivers and springs. Large scale motion of the sensor headassembly in the vertical or Z dimension approximately from zero to 20microns is controlled by a stacked piezo driver 304 mounted verticallyin a holder 306 preferably formed of an insulating ceramic such as thatsold under the trade name MACOR, available from Corning, mounted, forexample by an adhesive such as epoxy, in a portion of a piezo tubedriver 308, of the type which is well known in the art.

The upper end of the piezo driver 304 is preferably adhesively securedto the holder such as by epoxy, and the lower end of the piezo driver304 is preferably adhesively secured, such as by epoxy 307 to a sensorhead mounting block 309, preferably formed of an insulating ceramic suchas MACOR, to which the magnet of the sensor head assembly is secured.The range of motion of the stacked piezo drivers is of course dependentupon the piezo material selected and the length of the piezo stack. Thepiezo tube driver 308 is mounted to the lower end of the body of theinner sensor assembly, to provide for small scale x, y, and z motion ofsensor head assembly. The small scale motion achievable with the piezotube driver ranges approximately from zero to 5 microns in the verticalor z direction, and approximately from zero to 10 microns in thehorizontal or x and y dimensions, depending upon the size of thecantilever arms of the sensor head assembly, typically with an atomicresolution as small as approximately 0.02 nm in the vertical dimension,and approximately 0.03 nm in the horizontal dimension.

The scanning means preferably also includes feedback control means 310for driving the piezo tube in the vertical dimension as the probe tiptraverses the contours of the specimen, to maintain substantiallyconstant contact with a substantially constant force of the probe tipagainst the surface of the specimen. The control means is alsopreferably operative to drive the piezo tube to oscillate the cantileverwhen the probe is substantially in contact with the surface of thespecimen at a desired frequency which is a very high harmonic of thefundamental resonance frequency of the cantilever arm, and which ispreferably between about 0.1-2 MHz. The oscillation frequency iscurrently typically above about 0.4 MHz. For this purpose, the controlmeans can include, for example, means for generating about a 5 to 10volt sine wave signal to be applied to the piezo driver for control ofmovement in the z direction through a capacitor, such as a 0.1 mfdcapacitor (not shown). Alternatively, the oscillation of the cantileverarm can be driven by suitable control of the stacked piezo driver 304.

The control means 310 preferably comprises microprocessor meanselectrically connected to the photosensor means by line 313 to receivethe output signals indicative of deflection of the laser beam from theoptical lever arm means, and for generating the error signal indicativeof a variance from the constant amount of force of the probe tip againstthe specimen surface. The control means is electrically connected to thepiezoelectric drivers by control lines 314a-c for raising and loweringthe sensor head assembly with respect to the specimen for increasing ordecreasing the force of the probe tip against the specimen surface tomaintain the substantially constant amount of force of the probe tipagainst the specimen surface, and for raster scanning the sensor headassembly in a horizontal plane in X and Y directions. The opticallyencoded screw drive motors 217 of the motor driven legs are alsoconnected to the control means 310 by control lines 316 enabling thecontrol means to receive the optically encoded position signals from thedrive motors and to uniformly coordinate the operation of the drivemotors in raising and lowering the microscope.

In the method of the invention for resonance contact scanning forcemicroscopy, when the probe tip is substantially in contact with thesurface of the specimen, the cantilever arm of the microscope isoscillated toward and away from the specimen surface at a desiredfrequency in cycles of near and far excursions relative to the specimensurface, while the probe tip is maintained in contact with the surfacefor the specimen. The detection scheme is most effective at a highresonance harmonic of the fundamental resonance frequency of thecantilever arm, preferably between about 0.1 and 2.0 MHz, and currentlyabove about 0.4 MHz.

It has been found that this form of resonance contact modulation of thecantilever and probe tip at a high resonance harmonic of the fundamentalresonance frequency of the cantilever arm, is useful in reducing randomnoise at other frequencies. Experimentally, the resonance contact modeof operating a scanning force microscope has been found to reduce noisein images of specimens 3 to 6 fold, and streaking is greatly reduced. Inaddition, maximum feedback control settings before oscillation can beincreased up to two fold, allowing for faster scan rates for comparableimage quality. For example, in scanning a specimen in standard contactmode with a 75 micron scanner, with a scan range of about 50 microns ata scan rate of 4 lines per second, hydroplaning height variations ofabout 600 Å and noise in selected areas of from 30-50 Å can bedramatically reduced by scanning in a resonance contact mode at 473 KHzto less than 10 Å. In scanning a specimen with a scan range of 400 nm,with 15- 20 Å features, at a high scan rate of 5 lines per second with a1 micron scanner, noise of about 10 Å from scanning in a standardcontact mode can be reduced by scanning in a resonance contact mode at217 KHz to about 2-3 Å. Scanning in a resonance contact mode at 448 KHzover the region of the specimen further reduced noise to less than 2 Å.

The motion of the cantilever arm is measured, and a deflection signalindicative of deflection of the free end of the cantilever arm ispreferably generated by projection of a laser beam focused on the freeend of the reflective cantilever arm where the probe is located, whichreflects the beam to a photosensor which generates a deflection signaltracking the instantaneous deflection of the cantilever arm. Thedeflection signal is preferably used for generating a position feedbacksignal for maintaining the probe in contact with the specimen surface.The output topographical signal data may also be stored in a memorymeans (not shown), for use in displaying an image of the surface of thespecimen.

It will be appreciated that the apparatus and method of the inventionprovide for a resonance contact scanning force microscope which can beused for high speed scans and real time imaging of the surface of aspecimen with a substantial reduction in noise in specimen images due totip-surface interaction and acoustic noise, resulting in improvedresolution from both high and low speed scans.

It will be apparent from the foregoing that while particular forms ofthe invention have been illustrated and described, various modificationscan be made without departing from the spirit and scope of theinvention. Accordingly, it is not intended that the invention belimited, except as by the appended claims.

What is claimed is:
 1. A resonance contact scanning force microscope forexamining surface contours of a specimen, the microscope comprising:acantilever arm having a first free end and a second end secured to ameans to move said cantilever arm, said cantilever arm having afundamental resonance frequency; probe means secured to said free end ofsaid cantilever arm, said probe means including a probe tip adapted tofollow the surface contours of the specimen; scanning means for scanningsaid specimen relative to said probe tip; oscillator drive means forcausing said means to move said cantilever arm to oscillate saidcantilever arm at a desired harmonic frequency of said fundamentalresonance frequency above the cantilever arm fundamental resonancefrequency; deflection measuring means for measuring the deflection ofsaid free end of said cantilever arm and for generating a deflectionsignal indicative of an amount of deflection of the free end of saidcantilever arm; and probe position feedback control means formaintaining said probe tip in substantially constant contact with thesurface of the specimen in response to said deflection signal.
 2. Themicroscope of claim 1, wherein said means for measuring deflection ofsaid free end of said oscillating cantilever arm comprises laser lightsource means mounted to said body for producing a focused light beamdirected at and deflected by said free end of said cantilever arm,photosensor means mounted to said body for receiving said beam deflectedby said cantilever arm, and means connected to said photosensor means togenerate said deflection signal.
 3. The microscope of claim 2 whereinsaid light beam is a laser light beam.
 4. The microscope of claim 1,wherein said oscillator drive means is connected to said cantilever armand is operative to drive said cantilever arm with an oscillationamplitude between about 10 Å and 300 Å.
 5. The microscope of claim 1,wherein said oscillator drive means is connected to said scanning meansand is operative to drive said scanning means relative to saidcantilever arm with an oscillation amplitude between about 10 Å and 300Å.
 6. The microscope of claim 1, further including means to create animage of the specimen from said deflection signal.
 7. The microscope ofclaim 1, further including display means for displaying an image of thesurface of the specimen based upon said deflection signals.
 8. A methodfor operating a scanning force microscope for examining surface contoursof a specimen, said microscope having a cantilever arm and means to movesaid cantilever arm, said cantilever arm having a first end secured tosaid means to move said cantilever arm and a free end, said cantileverarm having a fundamental resonance frequency, said probe means securedto said free end of said cantilever arm and including a probe tipadapted to follow the surface contours of the specimen with asubstantially constant amount of force, scanning means for scanning saidspecimen relative to said probe tip, deflection measuring means formeasuring deflection of said free end of said cantilever arm and forgenerating a deflection signal indicative of an amount of deflection ofsaid free end of said cantilever arm, the steps of the methodcomprising:oscillating said cantilever arm at a desired harmonicfrequency of said fundamental resonance frequency above said fundamentalresonance frequency while maintaining said probe tip in substantiallyconstant contact with the surface of said specimen; and creating arepresentation of a physical characteristic of said surface of saidspecimen based upon said deflection signal.
 9. The method of claim 8,wherein said step of oscillating said cantilever arm relative to thesurface of said specimen comprises oscillating the cantilever arm at afrequency between about 0.1 and about 2.0 MHz.
 10. The method of claim8, wherein said step of oscillating said cantilever arm relative to thesurface of said specimen comprises oscillating the cantilever arm at afrequency above about 0.4 MHz.
 11. The method of claim 8, wherein saidstep of oscillating said cantilever arm relative to the surface of saidspecimen comprises driving oscillations of the cantilever arm at anamplitude between about 10 Å and 300 Å.
 12. The method of claim 8,wherein said step of oscillating said cantilever arm relative to thesurface of said specimen comprises driving oscillations of the specimenrelative to the cantilever arm at an amplitude between about 10 Å and300 Å.
 13. The method of claim 8, further including the step ofgenerating a probe position feedback signal based upon said deflectionsignal for maintaining the probe in contact with the surface of thespecimen.
 14. A scanning force microscope, comprising:a probe assembly,said probe assembly including a cantilever beam fixed to said probeassembly at a first end of said cantilever beam, said cantilever beamincluding a pointed probe affixed to a second free end of saidcantilever beam which is free to move relative to said first end, andsaid cantilever beam having a natural frequency of vibration; means toposition a sample in a fixed relationship to said probe assembly; meansto move said fixed first end of said cantilever beam relative to saidsample; means to measure the position of the free end of said cantileverbeam relative to said sample; means to position said probe in contactwith said sample with a substantially constant force; means to oscillatesaid cantilever beam at a frequency which is a harmonic of the naturalfrequency of vibration of said cantilever beam; and means to derive fromsaid measure of position of said free end of such cantilever beam andthe position of said sample a representation of a physicalcharacteristic of said sample.
 15. The scanning force microscope ofclaim 14, wherein said means to measure the position of the free end ofsaid cantilever beam further comprises:photosensor means operative toprovide an output indicative of the deflection of said cantilever beam;and focused light beam means positioned to direct a focused light beamto a reflective part of said cantilever beam and said photosensor meanspositioned to receive said focused light beam after reflection of saidfocused light beam from said cantilever beam.
 16. The scanning forcemicroscope of claim 15 wherein said focused light beam comprises a laserlight beam.
 17. The scanning force microscope of claim 14 wherein saidmeans to derive said representation comprises an electronic computer.